1. Field of the Invention
The invention relates to structures with artificially patterned refractive indexes.
2. Discussion of the Related Art
A photonic crystal is a material with a refractive index that varies periodically. In a photonic crystal, the optical response results from a coherent superposition of optical waves propagating therein. Photonic crystals with sufficiently high refractive index contrasts, e.g., a contrast of more than 2, exhibit complete photonic bandgaps. A photonic bandgap prohibits electromagnetic waves from propagating in such a crystal for wave vectors at optical wavelengths.
Herein, a photonic crystal with a complete photonic bandgap will be referred to as a photonic bandgap structure. Photonic bandgap structures have applications in a variety of optical devices. Thus, processes for fabricating such structures are useful.
One process for fabricating a photonic bandgap structure involves producing a periodic array of voids in a base material. In such a void-filled photonic bandgap structure, the refractive index pattern has a high contrast if the base material has a high refractive index.
Fabricating a void-filled structure from a material with a high refractive index typically involves a multi-step template-based process. First, a reaction step produces a template with a periodic array of voids therein. Second, a fill step introduces a material with a high refractive index into the voids of the template. Third, a removal step eliminates the template material from the filled structure to produce a porous structure of the high refractive index material alone.
Such template-based fabrication processes need to account for the properties of both the template material and the filling material. In particular, the filling step must not damage the template material, and the removal step must not damage the filling material. Unfortunately, many photoresists do not withstand the process of being filled with a material having a high refractive index. Similarly, many high refractive index materials do not withstand the treatments for removing templates made of photoresists.